/Engine/Source/Runtime/MeshConversion/Public/MeshDescriptionBuilder.h
Append a triangle to the mesh using the given vertex instances and PolygonGroup ID
[FTriangleID](API\Runtime\MeshDescription\FTriangleID) AppendTriangle ( const [FVertexInstanceID](API\Runtime\MeshDescription\FVertexInstanceID) & Instance0, const [FVertexInstanceID](API\Runtime\MeshDescription\FVertexInstanceID) & Instance1, const [FVertexInstanceID](API\Runtime\MeshDescription\FVertexInstanceID) & Instance2, const [FPolygonGroupID](API\Runtime\MeshDescription\FPolygonGroupID) & PolygonGroup )